Add category attribute with WYSIWYG enabled

I'm following this tutorial: http://www.atwix.com/magento/add-category-attribute/ All is working well, attributes are added to categories, but without the WYSIWYG button below the field. WYSIWYG is enabled in System > Config > Content Management.

$this->startSetup();
$this->addAttribute('catalog_category', 'custom_att', array(
    'group'         => 'General',
    'input'         => 'textarea',
    'type'          => 'text',
    'label'         => 'My attribute',
    'backend'       => '',
    'visible'       => true,
    'required'      => false,
    'wysiwyg_enabled' => true,
    'visible_on_front' => true,
    'is_html_allowed_on_front' => true,
    'global'        => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
));
$this->endSetup();

Whatever I try, WYSIWYG is not enabled for my attributes. Can anyone help? Or maybe there is a workaround for this?

EDIT: I searched other posts but all say that this code should add the WYSIWYG:

'wysiwyg_enabled' => true,

but it doesn't.